if you extrapolate the data from the dyno test, it appears that each "bolt on" results in a 20hp gain (approx)... the 340 is rated at 275hp and we've seen 320hp in "stock" dyno tests so let's split the difference and call it 300.
add an intake, headers and carb you're at 360. throw in a reasonable cam and there's 380. i'd wager to guess roller rockers, springs, and a sightly more aggressive cam has you sitting at 400.
all of that without touching the heads or running some wild compression number.
yeah, you can make 400 with 318 but it's all about compression and heads. with a 340 you already have that in place, so working with that potential should easily send it over 400 all day.
